The present invention relates to a speaker diaphragm and a manufacturing method thereof.
The present application claims priority from Japanese Application No. 2002-162972, the disclosure of which is incorporated herein by reference.
Speaker diaphragms are desired to have the following properties: High specific modulus and high specific flexural rigidity, high internal loss for the purpose of flattening the frequency response, durability to withstand mechanical fatigue, and resistance to various ambient conditions in which the speaker is used. To satisfy these requirements, various materials have been used for speaker diaphragms such as metal, ceramic, synthetic resin, synthetic fiber, natural fiber, and the like, and various techniques have been developed for better combinations or processing methods of these materials.
Speaker diaphragms made of synthetic resin can be manufactured with good productivity and have good environment resistance such as water resistance. Thermoplastic resin such as polypropylene (PP) that has high internal loss is used, for example, and it is the known practice to add fibers such as carbon fibers in the resin so as to attain a higher specific modulus for better sound quality.
One prior art example of speaker diaphragm made of fiber-reinforced resin is shown in Japanese Patent Application Laid-Open No. Hei 6-178385. According to this prior art, a diaphragm is formed by an injection molding or extrusion molding method from a resin composition including 100 weight parts of resin component including polypropylene polymer and polyolefin resin or rubber, 1 to 50 weight parts of glass balloon, and 5 to 30 weight parts of carbon fiber or carbon graphite. Further, this publication teaches that sufficient reinforcement can be achieved by using chopped carbon fiber of 1 to 10 mm filament length.
A common problem with diaphragms made of such fiber-reinforced resin is that an increase in the amount of fiber leads to a decrease of internal loss, making it difficult to flatten the frequency response, while a decrease in the amount of fiber in an attempt to attain effectively high internal loss will only result in a reduction in sound transmission speed due to smaller specific modulus, because of which favorable sound quality cannot be achieved.
Since specific modulus represented by E/ρ (E: elastic modulus, ρ: density) is a physical property, not only material but also structural factors can contribute to its improvement. For example, it is known that a fiber-reinforced resin diaphragm having a foamed layer thereinside has an improved specific modulus because of the decreased apparent density.
Forming such foamed layers in diaphragms, however, involves various complications: The fabrication process becomes complicated because of the necessity to admix a blowing agent in resin, or to perform fine control of pressure in molding. Foams reduce the mechanical rigidity of the diaphragm, and in some cases they may deteriorate its outer appearance.
Another problem with a foamed layer in a diaphragm is that while it helps attain a certain degree of specific modulus, it releases its internal stress, because of which the diaphragm's performance in sound transmission speed cannot be enhanced.